The intrauterine growth rate has become the standard to which postnatal growth of premature infants is compared, and the body composition of the full-term infant has become the standard from which nutrient requirements have been calculated. Metabolic balance studies, performed with premature infants to determine whether the amount of the dietary nutrient retained by the body per unit time matched the in utero accumulation of that nutrient over an equivalent period of time, have often demonstrated negative nutrient balances and have suggested body deficits of the nutrient under study. This has been particularly the experience with minerals such as calcium, zinc, and copper. Furthermore, due to technical difficulties that can either underestimate or overestimate both absorption and retention, metabolic balance studies produce variable results. Therefore in this investigation we propose to directly measure the bioavailability of dietary calcium, zinc, and copper in premature infants with stable isotope enrichment technology. Sixty appropriate for gestational age infants with birthweights less than or equal to 1,500 gm that tolerate full enteral nutrition of at least 100 kcal/kg/day by 7 days of age will form the final study population. These infants will be divided into 3 feeding groups of 20. Group I will receive a proprietary formula that has been specifically designed for the growing premature infant, Group II will receive their own mother's milk (PTHM), and Group III will receive fortified-PTHM (a mixture of their own mother's PTHM with a powdered mineral supplement). Ten infants in each group will have a birthweight less than 1,250 gm (Subgroup A) and 10 will have a birthweight between 1,250 and 1,500 gm (Subgroup B). Calcium, zinc, and copper bioavailability will be assessed by the addition of a solution containing tracer amounts of stable, non-radioactive, naturally occurring isotopes of those minerals, 46Ca, 70Zn, and 65Cu, to one feeding, followed by a 72 hour collection of stool and urine. Isotopic and mineral analyses of the stool and urine samples will then permit an accurate calculation of the fractional absorption of each administered stable isotope. Two bioavailability studies will be performed with each infant; the first between 10 and 14 days of age and the second at a body weight of about 1,500 gm for the infants in Subgroup A and about 1,750 gm for the infants in Subgroup B. Thus, this study will determine the effect of post-natal age on dietary calcium, zinc, and copper bioavailability, while comparing their bioavailability from PTHM, formula, and fortified-PTHM.